Regarding the team last night:
I'm glad you guys are working on new heroes, but I do want to mention that part of how poorly we were doing was because of our poor lineups every game. Obviously the main problem was going up against what seemed to be teams of five who were smurfs though.
Slardar: Fairly decent hero. He has a stun, but it's not guaranteed and pretty hard to hit a lot of times. His ult is good when you face invis heroes as well obviously. He really drops off if he doesn't get farm and doesn't help gank early on though. BKB is something you need up on him fairly quick in order to guarantee yourself in running down kills and engaging. Blink Dagger is the other super important item (generally before BKB) so you can be the initiator and guarantee your stun more.
Necro: I'm not a huge fan of this hero right now. I feel like he doesn't do much for the team unless you are grouped up early and he has an early Mek for big heals. His ultimate is really only good if they have a lot of strength/tanky heroes since it does damage based on the amount of hit points a hero has.
Nightstalker: NS is good, but you have to let him go mid, and I'm glad Nightz got some good practice on him doing that. But yeah, if a team picks NS that means your mid hero is chosen so don't pick another one like Zeus etc. I actually thought Nightz was doing fairly well last night. He was building good items, and he was trying to gank at pretty decent times. His main problem was that we just had poor lanes and poor hero choices (aka no lane had a guaranteed stun to set kills up for him). It's really hard to gank lanes when you don't have proper setup, and all that really means is that we had shitty stuns (just Slardar's). The one thing I would say to you Nightz is that at one point I noticed you had lots of urn charges banked up. You want to be using those as much as you can to heal allies or to cast it on a target you are about to try to gank for the extra damage. Urn is only good early on if you are maximizing your charge usage because eventually they are quite worthless once people get more HP and are higher levels. So don't be afraid to use them more.
So those three heroes above were our core team in every game last night. That didn't leave too much flexibility for the last two heroes since at least one of them had to be a pure carry. Also, if you guys are going into a game planning on playing a certain hero, there is no need to pick them immediately in All Pick. Obviously we were playing against some smurf teams because when they saw the heroes we chose they picked good heroes to counter them. So go ahead and just hold off on picking your hero in the hopes of the other team choosing some of their heroes first.
Splat: I know you enjoy Zeus and he can be a lot of fun (even in lane) when you are just starting out in the low tiers, but I would recommend getting comfortable with some more versatile supports like Lich, Tidehunter, Vengeful Spirit, Lion, Lina, Jakiro, and even Windrunner. Zeus is sort of a pub stomp hero, and you can't learn too much from him early on other than about how to kill steal with your ultimate which isn't that great of a thing haha.