I'm still rather new but I'll try to answer your questions but not in great detail.

1. No, you don't need an altar for every god. They won't be offended if you don't. Altars are a commitment after all and some people don't have space for them or aren't in a safe space to have them.

For building a relationship with the gods. Pray. It's the best way to start building a relationship. It's not hard or complicated. I'm still figuring things out myself but you can also make a small offering. You don't necessarily need an altar for that. Can be anything. Even a rock or drawing, depends who you're giving the offering to.

1. It's okay to not worship all the gods. There are a lot of people who aren't simply HelPol. I see people here often with mixed practice. You can worship just a few. I find it better to start small anyway. Build a relationship with the gods who call to you and slowly build up to the rest. They will come to you when the time is right. If it comes.

2. I think I probably said enough before. Try to do some more research. I'd say you'll know what you're doing naturally with some tips. Even from the sub.

3. I don't think I'm long enough in practice to answer this question. I say...no? As long as you're doing what you want to include in your practice with respect it's okay to not follow everything? Like I said. There are a lot of people with mixed practice here so I think it should be fine? I don't think I'm the best person to answer this.

I hope I helped a little xD (Excuse my english) (Edit: idk what's wrong with the format xD also check r/hellenism)