Firstly good on you for getting them out of there previous situation
I am sorry that one passed on it may of been something that he picked up from its previous situation.
The good news is the other one is eating, interacting with the other fish and not hiding to much, they do like caves and hiding out from time to time
As you rightly mentioned Clown Loaches are VERY social fish and they really do need there own kind to interact with!
Will he suffer from being on his own? only time will tell
How big is the little one?
How big are the other fish in your current tank?
what sort of filtration have you got in your tank?
You mentioned getting another tank in the spring is that a definite?
I ask as if you are definitely going to get a new tank and your current tank can deal with it ( someone else could confirm this as I do not know how to calculate volume of fish to size of tank I think it is 1 inch of fish to each Gallon of water in your tank but do not take this as Gospel. There are some calculators about on the web but I would recommend the advise of experienced aquarists against a rule of thumb calculation ( I have only been keeping fish for a year so do not consider myself experienced yet)) But maybe you have space to get a couple of friends for the little guy.
Yes clown Loaches do grow up to 12 inches but it takes quite a few years, but this size is not often reached (of course you have to plan for this as to the length of life they can achieve) through experience a couple of small 1 & half inch loaches are not going to grow that much in 6 or 7 months.
If he stays happy, eats and is active and your tank cannot take the extra load of 2 small loaches then just keep doing what you are doing
I have found clown Loaches to be a most entertaining Fish to look after and I am sure they will give you hours of entertainment with there playing dead and other antics.
I do recommend some serious research on this species as they do need specific living conditions.
This forum has some great info on Clown Loaches I would also recommend:
[URL="http/forums.loaches.com/"]http/forums.loaches.com/[/URL]
I do hope this helps
Regards onebto