Personally, I wouldn't stick anything with a male jag except a female in a 130. I know it is not what people want to hear but given the right care, jags are a very large growing cichlid and will easily use up that tank size for itself.
most people who try and put large cichlids together will try and grow them up together and even then it sometimes goes wrong. If that jag is 8 inches and from what you have written i presume he is in that tank on his own and has been for a little while, he is not going to appreciate company as he feels that tank is his terratoiry. But it is the same as anything in this hobby nothing is set in stone, he could be mellow with tankmates or not stand them it all depends on the fish's personality. Im not going to tell you not too put another fish in there if you have your heart set on it, just have a backup plan in case it all goes wrong. Keep an eye on them when there put in together, get a strong tank devider or better yet if you have another tank set it up just incase it goes wrong and be prepared to have to get rid of the newcomer and have your jag live on his own. I hope all goes well and your jag gets along with his new friend. good luck
You can try the 130 gallon,I have notice most tank dimensions are similiar to some of the larger tanks like a 180 gallon I have seen that was 72x18x24 or a 210 gallon that is 72x24x24.I guess having 6 inches more makes a big deal if you already have a tank that has the dimensions of length 6 inches shorter in width and same height.
A midas gets to be about 14inches or so. And they can be really aggressive. I have a midas that is only about 6 1/2 inches and he was in his tank with a jag that was a little over 7inches. They were doing just fine and one morning we woke up and the midas had ripped the jag to pieces. I guess he just decided over night that he did not want any tankmates anymore.
The tank needs to be rearranged and the light turned out just before u add a new fish. Leave the lights out for a day or two. Also feed both fish before introducing them to each other.
I use to have a 150 gal setup. Everyone told me a pair of midas or pair of jags would kill anything in the tank. I had a breeding pair of midas on the left side a breeding pair of jags i brought from florida in the other end. with. three catfish, uro, polleni sp, green terror, gold spangled pike, and some others i cant remember. never had one death from fighting only power outages. What im saying is it depends on your fish. If you have enuf to spread the aggression around. so no one fish gets picked on , then u can be fine,. But finding that magic number is hard, If your jag is small under 4 inchs. i would try other fish around same size. but i would add more then one fish. maybe 2 or 3. to spread out agression. with plenty of hiudeing spots