Hi cupman,
I have guineas and have found them to have good fertility in the temperatures that you mentioned. I would recomend against cross-breeding guineas. There are already many experienced guinea breeders creating new color varieties (the last count I saw was 34 different varieties) and I think that it is better to work on improving the varieties that already exist. In my own experience some mutts are nice, some are ugly, but most are so close to one or the other of the parent stock that most people would confuse them with purebreds. For this reason it is best to stay with purebred breeding. Breeding pens are always a good idea so that you can insure a good fertility rate with the proper male to female ratio and only breed the better birds (not the smaller or poorly looking ones). The only cross breedingI would reccomend is creating pied guineafowl. Many people find these to be very good looking. They can be obtained by crossing any white guinea with any colored guinea (although i recommend crossing the white with something dark such as chocolate, purple etc) not something light like buff duddotte or porcelein as it will be hard to see the pied effect. Be careful if you try to get pieds make sure that you have a white guinea, some of the light varieties can look like white guineas but wont produce a pied effect. If you think you have a white guinea look for spots on it. If it has any spots then it is not a white but some other light colored variety. Whites do not have any spots.
Good luck.