New to multiboxing, need some guidance

[Devile]

Welcome, glad to see more ppl interested in multiboxing. Now to your questions:

1. Yes it's easier to multibox with the same class. Just like u said, keybinding is a lot simpler but also, there are far less synergies. A mixed party with different classes is far better choice but can be challenging depending on the class. I've been playing with mixed parties for quite a long time now and I don't really find it that hard. Sure u have to spend a decent amount of time to define your build and keybinds, but after that, is not that complicated. Keep in mind that gearing a mixed party is a lot harder, reason why is a lot better to start with a full party of the same class in order to gear them all faster.
2. For beginners yes, its the best idea to start simple with 1 class. IsBoxer is not that complicated, but has a ton of features and trying to make things work for the first time can be tricky. Add on top of that the complexity of different keybind per class, will slow your learning curve considerably. Take your time with single class, get to know the IsBoxer (remap keys, round robin, target groups, etc.) and then try mixed parties.
3. WD is by far the easiest class to multibox. Sure 2.1 is pretty bad for petdocs, but still, T6 is very easy and relaxing. Yes, too many pets will lag u, but u dont need to have ALL pets from ALL WDs. Balance the amount of pets in your party, have other WDs without them and with more CC. Also WDs are easy to gear and pets do ok until Greater Rifts 30+. WDs can do pretty good with yellow items alone, so it's a good choice. DHs and Wizards are great for AoE, but those will require full 6 pieces sets in order to be that good. Without 6 piece sets, they will prolly fall behind WDs. You can also try a melee class, but dsync issue (characters running all over the place) can be a problem. Monks are in pretty good spot right now, but again, they need 4 piece set. To begin multiboxing, I still suggest 4 WDs. Get to know the software and then try other things.
4. Well, there's plenty information around here, in IsBoxer forums and Youtube videos. This forum is not that active, but still, the few of us who are active, have quite some knowledge and experience multiboxing, so we can help u out

I've multiboxed with all classes so far. Right now I'm doing 4x DH for speedfarming (Ubers, Key Wardens, Trial Keys, gems to rank 20 or so, etc.) and 3x Wizards + 1 WD for higher Greater Rifts. IMO, those are the most efficient classes right now.