1. Can a Monstrous creature put a tiny part of their base on a ruin and gain a cover save from it, even though its basically in the open (don’t need 25%)?
Ruins do not require that you be 25% obscured any more. If the terrain isn't on a base to begin with, decide pre-game with your opponent what qualifies as 'in' or 'out' of the ruin. If it is on a base, then so long as you're on it you're on it I'd say.
2. Can a small (infantry base) barley touch a ruin, but be completely in the open and still get a cover save as well?
Same as above, but if 99% of the squad is out and 1% of one model's base is touching you'd have a hard time convincing anyone you're in cover.
3. I think I know this, but does ignore cover sv. weapons, still ignore jink?
Jink is a cover save, so if something ignores cover it ignores Jink.
4. Do I now have to declare if I am going to jink, before he even rolls his shooting?
Absolutely. "The decision must be made before any To Hit rolls are made", pg. 167 BRB.
5. Can someone give me a breakdown of fast skimmer movement and still shooting?
Stationary and Combat Speed (up to 6") all weapons, Combat Speed (6"-12") two weapons at full BS and the rest Snap Shots.
6. How does fearless work in combat now? No fearless checks, and still no falling back or getting swept? (I always hated this rule, it’s the worst rule in the game… auto passing all these checks… make some armies just ignore a large portion of rules entirely.)
Fearless is an automatic pass of all Morale and Leadership tests, so no falling back and no sweeping indeed.
7. Can you run, then embark in a transport if in range?
You can only embark/disembark in the Movement Phase and Running happens in the Shooting Phase.
8. Does blast (large & small) hit multiple levels of a building, or just the level it was fired at?
Just the level it's firing at, though this one is a bit of a relic from 6th in my mind. I actually can't seem to find anything on this in the BRB. More research will be done...
My buddy I game with tends to move his units straight through solid building walls where there are no openings, a lot.
Difficult Terrain test and you're good in my books.
He also tends to place models in places (on terrain) where they can’t actually stand, and even fall over and/or move units though tight spots where model bases can’t fit. This is usually because he is hugging terrain.
If it can't stand, it can't fit. The only exception to this in my games is the occasional crater side during an assault, but we just push everyone up to the edge and say they're all in base to base contact.
so should I tell him not to do that stuff anymore and do I have any rules to back it up… he is a stickler for rules.
Have the rules to back up anything you contest, or shrug and roll dice on it. There are a few times my buddy and I have come up on things that neither of us are willing to move on. If neither of us have rules to back it up, we roll off and research later. It's a game and we only have so much time together to play it, we can do the rules work in between games.
Hope this helps!