I need Pokémon team building help (Gen's 2-7).
As the title says I need help building a Pokémon team for generations 2 and onwards. I'd like to know if from generations 2-7 should I keep my Pokémon all generation exclusives and/or should I have previous generation Pokémon in my team as well please and thank you in advance. 

Do you need help with a team consisting of pokes from gen 2-7 or you need a team for every gen?
If you need a team for every gen this is my recommendation for Gen 2 :
So, in building a pokemon team, i like to think it like overwatch you know, there's the tank, the healer and then the offensive people (but in this case, im using 2 tanks lmao)
-Pikachu (because it is super fast and have the ability to giving a chance for the foe to be paralyzed and you can double team a lot to dodge attacks)
-cloyster (because it can learn shell smash which is a move that will make it super strong and then you can use icicle smash which is a multiple attacking move)
-snorlax (because it has a lot of hp and a good defense or sp defense attack (?, i forgot lmao) so, he's the tank of the team, he the pokemon's Reinhardt)
-Blissey (because it has a lot of hp and between defense and sp defense stats (i forgot sorry) so if your offensive peeps is hurt, you can choose between snorlax and blissey to tank the hit while you healing them)
-Dragonite (because this pokemon is the best spc attacker imo, especially when you apply a good moveset to it. And find the one with the multiscale thingy (its an ability that made your pokes will not die in one hit))
-gengar (another great attacker, use the combination of the sleep move (its called hypnosis i think i forgot) and sleep talk, and you'll destroy the foes)
I hope this help, if you need the help with the other gen, I'll help you but i need a bit of research cause gen 2 is the gen that i know the most of

Well it all depends on your preferences if you want to keep them generation specific or not, but me personally just use straight Gen one pokémon in my first run through other than my starter. A good rule of thumb is always to have a party of mixed types.

I always like to use all new pokemon in my teams every gen, to get to know them better and see which ones I like.

9 times out of 10 I go with the grass starter and build my team from that.

There's one thing when it comes to making a team that many people overlook. You'll need some kind of core. The inner, most important circle of your team of 6, if you will.

Because you receive a starter pokemon at the beginning of the game, many people use a fire-water-grass core, which can handle a lot of types already. 

A flying type is always recommended as well.

For my 5th pokemon I usually go with a psychic / electric / ground type. They cover a few of the remaining weaknesses my team has.

And my last is usually filler. Sometimes it's another one from the three types listed above, sometimes it's something random like a steel or ghost type. Basically if my first 5 choices have a glaring weakness then I use the 6th slot to counter that type. 

In my opinion, building a team in pokemon is a hard thing cause you have to cover all the enemy's pokemons type and also choosing the right pokemon with the right statistic, like, if you want to have a good pokemon, you have to get the right nature, ability, and with the high IV's, and after that, you need to EV train them too.
So, if you build a pokemon team based by the types only, you wont have a great time battling the elite four, because their poke's are perfectly EV and IV trained (if you dont know what EV and IV are, bulbapedia have a post that talks about it). So, its not all about types in my opinion, its about how you train your pokemon and the movesets that the pokemon have. For example, a Blaziken. Blaziken's move is mostly a Physical Attacking moves, and if you killed a lot of abras and magnemite (which yields Special attacks EV) in the process of leveling it up to 100, you're basically wasting the potential of your pokemon because you're adding the stats thats not needed for the pokemon because blaziken is a Physical attacker, and by adding special attack evs is a super bad move.
So thats my example, building a pokemon team is all about training them, like, if you have a rayquaza with a lot of powerful move but you EV trained it to increase HP and Defense, that rayquaza is basically useless. So think about your team wisely and train them nicely, dont kill random pokemon in the training process, go to bulbapedia and search 'pokemon ev yields' and learn about EV and IV and pokemon traits because thats the key of a great team