I have never done this questline before and was wondering if I need certain skills to do the missions.

Do I need a high level in stealth? What about pickpocket? I know its called the THIEVES guild, but the companions and the college didn't really require those specific skills to be able to do the quests. I'm also using heavy armour, if that makes a difference.

Thanks for reading!

In short, no. Beyond the initiation, all you need is a decent skill at killing people. Sneaking can help you avoid killing too many people, but you're probably still going to kill a lot more stuff than the thieves' quest-line in Oblivion.

Are there swarms of enemies? Are they strong? Do I get a bounty for it?

Bethesda stopped requiring characters to have guild-requisite skills in Oblivion. It will probably never return.

despite all that, you should level up sneaking. the questline is much more enjoyable and less frustrating if you have a good sneak skill. especially if you wanna do those side jobs.

Swarms? No, you'll most frequently be working against one though there a few missions taking you into dangerous areas. That is of course unless you alert guards to your activities and wish to avoid jail. Stealth is recommended and you can get your stealth to a high level wearing heavy armor, it would be recommended taking great care while doing the bulk of the quests and muffle spells, boots can help immeasurably to this end.

Picking locks needs no perks and when your stealth is high enough picking pockets is problematic, you don't need any perks in that as well unless you want to put a potion in someones inventory and have them take it.