Based on your stock, I'm going to say that this combination will fail because of most of your stock is incompatible with a gar:
- Monos and oscar
- Cichlids and gars generally don't mix well especially if they are aggressive cichlids as these two species can be.
- TSN and channel cats
- These catfish will outgrow the gar very quickly and will probably try to eat it (especially the TSN).
- Silver arowana
- Arowanas and gars usually do not mix will since both fish can spook easily resulting in injury and/or death to both fishes. Gars also do not get along very well with shiny silver fish.
- Pleco
- Plecos may suck on the gar's slime coat which will allow for the gar to get an infection that will more than likely prove to be fatal.
As for the CK and turtle, I don't know what to tell you. If the turtle doesn't bother the current fishes, then I don't see why not. I'm not too sure on the CK.
Now then, many people will say, "O, one or more of those combinations worked for me," and other stuff. Please remember that these are
exceptions to the rule(s) rather than norms.
EDIT: Crap, I must be tired from work today, and I missed the most important reason why this will not work:
TRUE GARS ARE ILLEGAL TO OWN IN CALIFORNIA!!! They are illegal despite the fact that many LFS's there defy the law and still have them for sale. Please remember that important piece of information.