Hi
If Daniel had artistic tendencies then as yet it isn't obvious from the records that his employment had any connection with art.
If Thomas senior was serving in the army that may explain why two of his sons give Chelsea as their place of birth and why his wife was born in Ireland, though if he went to India you might have expected him to have served in the East India Company army and not the regular army.
Evidence of only one East India Army cadet called Thomas Mills
Surname Mills
Forename(s) Thomas Milliken
Season(s) 1819-20
It doesn't look likely that this is him. Googling that name brings up records on a solicitor. The middle name is quite distinctive.
If the sons were baptised in Chelsea before an army chaplain the records will be in the GRO army registers (overseas section whether they happened overseas or not). Findmypast holds the GRO army registers. The missing baptisms however might be because Thomas' wife Mary was a Catholic?
Thomas certainly isn't in the cadet registers for Sandhurst
http://www.nationalarchives.gov.uk/records/looking-for-person/britisharmyofficerbefore1913.htm?WT.lp=rg-3140It is much less likely he was a 'gentleman' if after his army service he was a porter. The definition of gentleman is someone who doesn't have to work for a living. If he came from such a family background there is no evidence to show he and his family lived anything but a working class life.
Gentleman rankers
http://en.wikipedia.org/wiki/Gentleman_rankerIn the early C19th it would have been harsh and grim to serve as an ordinary soldier, enlisting as a private.
Regards
Valda