Getting a 300 gallon! That is a fantasy tank for an African Cichlid community, or just about any aquarium fish community.
My Fusco is about 5.5" inches & is the dominate Hap in the tank.
I no longer have a Venustus, but the Fusco never really bothered it. Although fairly aggressive, the Fusco is no match for my Zebra Chilumba or Red Zebra.
You have a very good stock list. Keeping them together & healthy in a 125 might be a challenge, but is certainly doable. Especially in a temporary situation.
Be a good idea to start rock hunting at your local rockyard or landscape company. There is a lot of space to fill in a 300 gal. tank. Much cheaper & huge selection at a rockyard.
A 300 gal. tank with a good sized rock hardscape & plenty of open swimming area above, allows for a mixed community of Haps, Peacocks & select Mbuna. I really like my Haps & Peacocks, but the tank would be lacking without the Mbuna dashing between the cracks & crevaces of the rocks.
Just keep in touch with the forum, we'll try to help if any problems arise.