I know it's wisest to build your own island or what not to do your first house or dungeon, but if I wanted to build a cottage somewhere on the mainland - how do I ensure that no animals are scripted to randomly spawn there? And if they do, will they show up inside my house or will it cause crash issues?

I've always wanted to build a house on the pencsula north of Pelagiad (the one on the west side of Lake Amaya) but I'm pretty sure a rat or something pops there.

There's several creature spawns around and on that peninsula.

When you view the area in the CS render window, you will see little monkeys (No, I'm not taking the piss) dotting the landscape. The monkey's are the spawn points. Double click them to open the spawn points properties and you can see the leveled spawn list.

If any spawn points are in your way, select and delete them. Alternatively, you might change them to only spawn some of the more docile creatures and move them out the way. If you leave them there, they won't show up inside the house since the house interior is a separate cell linked to by load doors. If they spawn inside the exterior house model, they will be able to walk out since collision is one way.

I assumed that's what the little ninja monkeys were. lol Deleting some random spawn isn't going to cause issues with the game, right? As far as I can remember, nothing important (ie: for quests or such) spawns in that area.

No, you won't do any harm by deleting the spawn points. Your save game, however, will probably still recognize the spawn points after you've deleted them, but give it a few in-game days and things should stop spawning there.