Well, you're asking the gar forum, so many people will say yes, including me... I'm not into koi. But you already have the koi, and koi ponds can be beautiful. your call bud.
Gator gars are fine to mix together as long as sizes are similar. I currently have 6 together ranging from 19" to 34". There was quite a bit of nipping the less gars there were in the group.
Gators can be mixed with other gars, you have to keep in mind the different growth rates and the more aggressive nature of the gator though.
Lastly I would suggest going with a smaller gar species, gators are not easy to care for and are expensive. I'm currently spending about 10 dollars a week just on food for mine and they are all still babies.
The money won't be an issue I already spend $50 a week feeding my other critters I plan on feeding him fillet when he's over 3ft every other day and so when my fl get there full grown size I will introduce the gator gars then if that's the case if there's aggression issues this spring am making a 700g pond (indoor) I'll put them in there
I would go with a "school" of floridas personally. usually you look into a pond and see koi.. i think a group of gar, and with the top view of the beautiful unique patterning floridas get would look very nice.