Interested in this topic based on some claims made in the media recently. As follow-ups, what were the federal government's (or state governments') practices when it came to the use of slave labor for public projects? Were there publicly-owned slaves, or were there companies who owned slaves who could be contracted?
There's a lot of discussion on this thread, from the front page of the sub, that you might find interesting.
I can't really answer your question about the relative comfort of these particular slaves, but what I can tell you is that when people ask this sort of question, the response is often that the question is irrelevant, even offensive. Why? Because once you have taken the step of enslaving another person--literally denying them their very basic rights as a human being--then no conditions under which they live can be considered "decent." While better food and shelter might have made some difference to some of the individuals in question, to ask this sort of question and parse these sorts of issues is in its very nature to obfuscate the absolute horror, the total black stain upon decency, that was chattel slavery in the eighteenth and nineteenth century United States.
Like /u/pipkin42 pointed out, the thread they linked has some great discussion about slave conditions during the construction of the White House. And I 1000000% agree with their point about talking about emphasizing minimum food and lodging for slaves. But you've also asked some questions about slave hiring and government ownership of slaves.
I've personally never seen anything saying that the federal government owned slaves. But as the White House example illustrates, they had no problem with commissioning slave labor. Commissioning slave labor was also common for state governments, except of course in states where they had abolished slavery.
It was common practice for slave-owning planters and organizations to supplement their incomes by renting out their slaves for hire. This would of course often be to do things like construction, but also would include fieldwork, mining, and domestic house work. It was common enough that standard rental contracts and insurance agreements developed for rented slave labor. Rented slaves in rural areas were treated largely the same as non-rented slaves, though there's some debate about how much autonomy hired slaves had in urban areas. [1][2]
You also asked about state government ownership of slaves, and here there's clear evidence that they did, primarily slaves received as escheat (the owner died with no heirs, so ownership reverted to the state) or slaves seized as contraband. Often they were immediately sold at auction. The University of North Carolina has done some interesting investigation of their early funding from sales of escheated slaves[3]. Slaves were seized as contraband on ships after the ban of the international slave trade went into effect in 1808, and ownership of them would revert to the state government closest to where the ship was seized. So a slave ship seized off the Massachusetts coast where slavery was abolished would result in the slaves being freed. But if the same ship were seized off the coast of Louisiana, the only real difference would be that the revenue of their sale went to the Louisiana state government rather than the smugglers that shipped them.
Sources:
[1] - Zaborney, John J. Slaves for Hire: Renting Enslaved Laborers in Antebellum Virginia. Baton Rouge: Louisiana State UP, 2012. Print.
[2] - “Hiring of Slaves” in Junius P. Rodriguez, ed., Slavery in the United States: A Social, Political, and Historical Encyclopedia
[3] - SLAVES AS ESCHEATS. University of North Carolina, n.d. Web. 27 July 2016. https://exhibits.lib.unc.edu/exhibits/show/slavery/escheats