Starters would be easy. Prof from whatver region you start with gives you one you choose.. And if you ever want another one from another region just tell about how another Prof lets you choose one of three in exchange for some help.
I personally don't see a need for fly. I would much rather travel on foot/bike around the countryside, taking in the landscape and the natural environment. It would definitely be helpful, but not necessary. Even something like surf could be questioned, as you likely could get to island cities by boat. It would still be needed for getting across rivers or to obscure locations, though, so I'd probably have a surfer on my team. Those are primarily navigation roles, and we can get around fairly well in our non-Pokemon world anyways, so while they would be helpful they aren't the most important. Fighter/Tank/battle roles are more subjective then anything else, depending on your goals in the adventure. If you are trying to collect the badges and compete in the Elite Four (or a tournament similar to the anime) you would probably care much more about team composition than someone who wanted to travel across the region simply for the experience of growing and surviving with your Pokemon. Cleric does seem important, though, as injuries will almost certainly be had and everyone will likely get tired.
I think some of the most important roles for Pokemon involve protection against weather or nature. If you travel somewhere cold, having a fire type Pokemon will be invaluable. The reverse is true if you are traveling somewhere really hot, and you need water to cool down in. A fire type (or something with a fire type move) can help navigate dark caves or at night, while something with defog or sunny day could help clear up unfavorable weather. While you would not necessarily need Pokemon with clear, competitive-based roles, you will likely still need something to help protect against wild, untamed Pokemon that could try to steal food sources or drive you away from nests you unwittingly stumble upon. You would need protection against rival trainers or thieves as well. This role could theoretically be filled by any Pokemon, however, as long as they were strong enough (which should come naturally over the course of the journey).
Besides that there are additional utility needs represented by the remaining HM's. Cut, Strength, Rock Smash can be used to clear paths or build shelters. Something with harvest could provide primitive food sources. A Pokemon with flash or a bright fire-type move to signal your location in case of an emergency. Something with electric powers to recharge the battery of a phone, pokegear, or pokedex (which will be important to keep functioning while far from society). I don't know how clean the water from water Pokemon is, or even if it's drinkable (I would be disgusted tbh), but if you desired you could keep something around to serve as a source of fresh water. A nocturnal Pokemon to keep you company while the others rest, or to keep lookout at night. A flying Pokemon to scout paths, look for help, etc. Something to possibly ride on if you get tired but can't stop for rest, or to help carry supplies. Something fast to run ahead or transport you quickly. I wouldn't mind something with dig, to create underground shelters which would feel safer than being exposed on the surface.
Really, there's so many different roles Pokemon could fill that it would be really hard to hit them all. I would just suggest selecting your favorites to travel with. But even then, in my opinion finding new ways to overcome obstacles with your favorite Pokemon is part of the fun and a crucial aspect of the experience.
So yeah, here's the sort of team I would have. Seviper would be my starter, probably start off as a kid, and would grow up pretty quickly into a beastly fighter. I mean, while it's weak in the games, in real life this thing would be a monster on the battlefield.
Cacturne would be my go-to guy for whenever I'm in trouble. Since I live in BC, it rains quite a bit around here, so Cacturne's water absorb would work perfectly.
Mightyena is just beastly looking, and would probably be an efficient tracker, and would most likely be out of his pokeball for protection and such.
Sceptile would be my powerhouse with attacks like leafblade and such, and could give most people a run for his money. Plus, with his speed he could help me catch other pokemon, or scout the area for danger.
Wailmer is my surfing buddy, and it's large size would make it a sizeable tank on the battlefield, giving me a good advantage in battle against smaller foes with bounce.
And skarmory is my tank and flyer. Them wings could probably deflect most things, and he could fly me around at a good speed to various cities.
Persian would just be my buddy, heh. Basically my pet cat, but I'd still send him into a fight he could win... Otherwise he'd be more of a companion than a battler.
Cloyster, on the other hand, would be my scary-as-hell powerhouse. Cloyster and Persian will always be my favorite two pokes.
I love supportive Pokemon, especially those that can really frustrate people, so Liepard would definitely be on my team... Plus, I just like having cats around.
I would have a male Swanna just because it's such a feminine design and I don't like those tertiary sex characteristics... So mine would just be a drag queen. Also, my flier and surfer. Cloyster doesn't have time for that, he's much too busy being terrifying.
Putting things to sleep is one of my favorite things (I greatly despise the sleep clause), and in real life Hypno would undoubtedly be the best at doing so. It's the whole point of his existence. Plus, I could help get this wayward pedophile on the road to being a functioning member of society.
Such a well rounded pokemon, which I love. My favorite eeveelution by a mile.
seems to me like psychic types could function as flyers - they can levitate, right? who says they can't do it for long distances? plus some learn teleport, though i'd think pretty much all of them could do it. your sisters seem pretty cute, though.
Yeah, I also figured that Teleport would be just as legitimate a form of transportation. On psychic types pseudo flying though, I don't know if Telekinesis depends on your distance from the object you are propelling yourself from.
Well, they are 10 years old so they are actually pretty cute.