I think it makes more sense for them to both be singular, since each question should be limited to one topic of recommendation per question. But the case could also be made that questions using these tags are seeking multiple different recommendations for answers in some cases, so it's not a big deal if they both have the plural "s". Either way, it annoys me that they are different :P
software-recommendation — DONE.
A synonym isn't necessary here. Synonyms were intended to link two completely separate words meaning essentially the same thing (think 'car' vs 'auto'). For simple variations on the same word, text completion will help guide the user to the correct usage.
soft → software-recommendation
There's no need to bulk up the tag listings with every word inflection.