Hey, don't apologise - the more information we have, the better it can help us offer our thoughts on what you can do to make a new addition to your family work. And it's not a bad thing to keep your cats as indoor only. There are many different ways of keeping happy cats, and the indoor/outdoor debate depends very much on personal circumstances and the characteristics of your cats (your two sound lovely, and a great age for adding a new cat
)
My own thoughts would be that it's great you're moving to a larger place, as it will give each of the cats more room to spread out if they choose. It will also help if you can put various cat climbing platforms around the new house, so each of them has a place to go (although having regard to the rats too, maybe not every room in the house
).
From the sounds of your two, I would maybe think about getting another relatively young cat - young cats usually respond well to cats slightly younger than themselves, say between 6 and 12 months, and still with a kittenish streak, as it appeals to their playful side. An older cat may be less well inclined to adapt (although that's a generalisation - plenty do quite successfully.)
How long will it be until you move? If it's not long, I'd maybe think about increasing your family when you move, so they all get a chance to investigate the new spaces at the same time. This may work especially well if you can squirrel any new member of the family in one of the empty rooms, so it appears like the cat came with the house and is a great source of wonder.
This lady has some really terrific advice on cats, and you may find this helpful:
http://www.wayofcats.com/blog/why-third-cat-is-different/5877She also talks about different cat types, and the chances are you'll recognise Calcifer and Lux from the descriptors, which may help with your choice of a new family addition. There are all sorts of useful hints and tips, many of which I've tried myself and been chuffed to bits with the results, so check them out.
Hope this will help.