Translation:I will have dinner with some colleagues later.
Nowhere does it say how many colleagues so 'some' isn't necessary beforehand
Why does colleague have to be plural? There is no indication that there is more than one colleague?
Yes, and while in Beijing accent it'd mean that (yi2 huir with hui4), 一会儿 (yi4 hui3 er2) means 5 minutes.
This sentence feels a little off , I'd add a 过 as in 我过一会儿会跟同事吃晚饭
Correct me if I'm wrong, but I never heard 一会儿 without a verb like 过，等，睡 etc etc.
過一會=等會=一會=待會=一下子 In Taiwan, 等會 & 待會&一下子 are used more often. But anyway, with 過 or not it doesn't matter, all means later.
But if you add verb, the meaning is different. 睡一會 sleep awhile 等一會 wait awhile.
一会儿 can indeed occur by itself as a Time Word. I should add that it always refers to a relatively short period of time, so "later" is not the best translation. "After a little while" would be a better match as rcc002 notes below.
Technically yes. But in China, words like 一会儿 and 马上 have been abused (sadly)--if you hear people say something like 我马上/很快/一会儿就到, don't be surprised if they show up an hour later ;-) Instead, ask them to specify an exact time (e.g. 九点半到).
Both are fine. Maybe technically it should be 过一会儿, but in China, it's very common for people to omit 过. You may also hear 过会儿 sometimes.
There isn't " my (colleagues)" in the English sentence. It's just tongshi
I tried "I will eat dinner with my work mate in a little while." Just wondering why everyone else (seemingly correctly) got the plural from 同事. There seemed to be nothing in context to suggest many vs one.
There should be no 'some' as no its indication on the Chinese sentence.