Names

I'm writing a story, and I made up the father-names of Elrond and Elros to be Findelum and Fingemen respectively. Findelum comes from (fin-) as in Finwë, for his heritage, and telumë, which means sky or heavens, so his name would be Fin of the Skies. I thought this would work because the name Findegil softens the t from tekil. It could also mean dusk-tressed, for his black hair. Fingemen again comes from (fin-), plus kemen, as in earth, so his name would be Fin of the Earth. Do these work?

I guess you are trying to form Names in Sindarin? In Sindarin old telumē would develop to telu since m first becomes v after vowels and then v was lost before u, so Findelu. Similarly the Sindarin cognate of Quenya cemen is ceven, so Fingeven. I would advise you to directly search for Sindarin words instead of picking Quenya words and speculating on cognates that can get quite complicated. If I have misunderstood you and you actually want Quenya names, then all the soft mutation and vowel loss should not happen: Telumefin (Telumefinw-) (but be aware about the similar Telufinwë) and Cemnafinwe (cemna “earthen” to avoid the sequence -nf- with cemen).