RFC 1912 define a valueset between 20 minutes and 12h, but I have never seen a system with values under 1h (perhaps because a few systems ignores values under 1h).
I've never seen a system ignoring the TTL. TTL below 20 minutes is not uncommon, e.g. all dyndns-services use very short TTLs, e.g. 60 seconds. This is not a problem at all.
Regards Michael