I'll say welcome too. Sorry I didn't manage to get to you a couple of days earlier. I would have pointed you in the direction for substrates, water parameters, etc.
I take it by 75g as a start you've got designs on MTS and going bigger in the future !! (That's the way we've all done - I suspect)
Seriously 75g will be fine for them, I take it your new fish are Mbuna ? It tends to be less about no of gallons with Mbuna and more about tank surface area and numbers and size of rock piles. Most tanks are proportioned such that they aren't too high. I once stocked a 120g with Malawis and had to stock less than I could for that tank due to it being a deep tank.
In a tank your size, you'll probably end up with the "master of the tank" and then 2 lieutenants who command a rock pile or half of the main pile each. After that it's about who's allowed to live in their territories. If you're happy to keep up with the water changes and it is mbuna, go for 1 fish per 2g. If it's another type, then let us know and we'll see what we can do for you.
As for getting the fish in there now, can I just ask how long the filter was cycled for and how ?
The normal thing for Malawis, with trying to get numbers into the tank quickly, is to carry out a fishless cycle so that the filters are capable of accepting a high bio-load straight away.
As for plants - see what you can get away with, Vallisneria like the harder, higher pH water and tend to get left alone. Generally speaking try to find something that will grow quickly and have lots of it. One or two plants here and there will soon get seen as the "salad bar" and these guys like their greens !!