Can a low fee unconfirmed transaction prevent confirmations of following transactions?
Yes, if the second transaction spends the output of the first one, then #2 cannot be confirmed until after #1 is confirmed.  (Though they can be confirmed in a single block, as long as #1 appears before #2 in that block, in which case the confirmations are effectively simultaneous.)

Most miners will consider transactions on a "child pays for parent" basis, so that a low fee for #1 could be compensated by a high fee in #2.  The two transactions would be considered as a single "lump" with their combined fee, and a miner would confirm them if the combined lump pays a higher fee than the next most lucrative transaction (or lump) that could fill that space.