Skirmisher

Hello aow4 enjoyer!
So I love playing with skirmisher Units and bought all Expensions for the Reaver Faction and for Tome of the Stormborne.
I pretty bad at combat, so my skirmisher sadly die alot, also my knowledge of the game is not that great.
I have four questions:

1. I saw units like the slither hatchling can evolve into normal slither, how do I level them up? Do they need more exp.? And if they have enough exp what do I need to do?
2. How do my Heroes get the Hero Skill Wyvern Keeper?
3. Does it make sense to give my Hero a Special Mount for the Dragoon Units, or do I just try to switch to the Stormbringer Unit?
4. and most importent - with which classes would you recommend to combine with my skirmisher? I think Shield make sense but I would like to hear from experienced player.
   BTW: I dont really care if this is a good idea or extremly awful, just let me know what you would to in my seat. It is just for some chilled singleplayer games, when I play with my friend we always skip combat.