Yes, there's a rule for neuter adjectives with roots ending in a double consonant: when the -t ending is added, it replaces the final consonant.
glatt -> glatt (phew, no awkward triple t)
trygg -> trygt
Exceptions are made in cases where removing the double consonant would make the word identical to another neuter adjective (one whose root ends in a single consonant):
ful -> fult
full -> fullt (!)
I actually can't think of any other examples, so you needn't worry too much about running into exceptions you don't know about.
someone correct me if i'm wrong, en is for masculine nouns and et is for nueter nouns