I would not say that 12 inches are too short ... but the longer the better ... most of the ones available commercially are generally 24 inches or longer on the rear ... and 30 inches and longer on the front.
I just received a 72 Chevelle chassis, it has a triangulated 4-link arrangement.
The upper arms are fairly short at ~14". My 34Ford PU project has a triangulated 4-link and the upper arms are ~12". I kind of thought those are a wee bit short due to the vehicle travel of the back end. I will definitely be installing both longer upper and lower arms in the future. 12" just really makes for a rough ride with short travel arms. My 2-cents worth.....lol....
If the instant center is far enough forward, you wouldn't have to worry about pinion angle changes. If this was for a competition-only SCCA autocross car, the length might be quite acceptable. But, for a softly sprung street rod, it's a wee bit short.
(I'm assuming all links are this short. The 2005+ Mustang 3link has an upper link which is quite a bit shorter! Not crazy about that, but the Ford engineers seemed to think it was acceptable.)