I doubt the senegal will ever by large enough to live with a gar. Picture the gar hollowed out, If you think the tankmate could fit into that space then the gar will think the same thing !
I've seen young ones cannibalize other gar's who were almost identical in size. GAr's will do fine but you'll want large growing species that will remain similar sized or larger then the gar's.
I'd avoid anything long and slender build unless it's equal sized or larger. Avoid anything aggressive and anything with a sucker mouth as mentioned above. tall bodied fish is a good suggestion as well, harder to swallow less tempting to try.
Other species of Bichir will work, as long ad they are larger species who will always be equal sized or larger. Other bottom dwellers like catfish or clown loaches are options, again as long as they are large enough not to be tempting prey. Dat's, or Pearch species. Larger peaceful cichlids are all avenues to take. They also do well in sub tropical setups, opens the doors for a few other interesting tank mates that arnt commonly seen.