TBH dude, theres no need to come on here asking for what your going todo and ask for views and advice then completely lare off betta. Granted what he said.
A 5 Gallon marine tank is very hard to maintain a stable marine environment then you think. Freshwater is completely different to your 14 gallon.
This is because you have got to maintain a good constant PH, SG, temp, ammonia,nitrite and nitrate which is very hard in a small amount of water.
As for your claim that if the fish aint ill or dead its not stressed well you will find you can tell, how?
1. colour of the fish
2. Activity of the fish
3. feeding
4. gasping for air is even one!
With these points there will be a high chance that 98% marine fish part from inverts such as crabs and cleaner shrimps will become stressed, leading to ich, to death. So you would of just wasted your money.
If you want 3 fish you would be better off with a 20 gallon min its then easier to maintain and wont cause as many issues.
You can do what ever you want but it will only end in you loosing out trust me.