I have a jag about 4-5 inch in with 4 oscars ( 2 smaller then the jag and kick his ass) flowerhorn, red terror and sds. The jag is most likely the weakest fish in that tank now that the angels are out, I think jags are all talked up even my breeding pair with fry dont come any where close to my older flowerhorns.
My jacks are a bit smaller and share a tank with umbee,choco and a con and one jack is a bit of a fighter but he always get told off by the umbee.
Id say you would be fine, people over here even keep jags and jacks in oscar/ serv tanks all the time and it goes fine long as the tanks not a midget. They mind there own business more and wont attack anyone else they come to there little area or hits them first from what I have seen, else you have 2 jags that causes some issues. you wont be able to find out until you try it, Maybe I have crazy luck but I have 5 inch vieja living with a 24 inch jardini and I bet if you asked that Q here everyone would say no lol.
a 24 inch jardini with a 5 inch vieja..damn.. my jardini is just abt 11 inches and it recently killed a 6 inch oscar..they r supposed to b really agressive.. has ur jardini been used to staying with tankmates since u bought him?