Yes, It is true that Rig Veda consists of 4,32,000 syllables. It is verified by Shatapatha Brahmana in 10:4:2:23
He arranged the Rik-verses into twelve thousand of Brihatîs , for of that extent are the verses created by Prajapati. At-the thirtieth arrangement they came to an end in the Paṅktis; and because it was at the thirtieth arrangement that they ...
Seven is significant because during the onset of creation, seven was the number of surrounding sheaths in the Yajna (sacrifice) in which Purusha himself was the offering. And, the number of such offerings (Samit) was also 21, a multiple of 7.
The relevant mantra from the Purusha Suktam is the following:
SaptAsyAsan paridhayas (1) Trih sapta samidhah ...
I'm not answering about 13 because i'm unsure about it either being holy or unholy.(So, treat this as a partial answer)
However,I don't think that the number is 8(ashta) is inauspicious in Hinduism.
Because,we have many important(&auspicious) Hindu facts and facets related to it.
Few are given below:
The eight limbed path of Yoga ...
Refer the numbering system here comparing Sanskrit and western numbering:
शङ्कु (śaṅku) 1,00,000 koṭi 1012 10,00,00,00,00,000 One trillion
Perhaps what is stated above in your question is an exaggerated figure, what we normally term as "Atishyokti".
It's just like when we ask some one how many people came to watch the match, so if a person ...