Does anyone have any tips on how to practice this sort of thing?
When practicing my scales in triplets or 16th note triplets I can lock in straight away but I’m having a hard time not rushing after the pull-off when I try to play in 16th notes
Any help is greatly appreciated 
depends on the phrase you are meaning to play against the rhythm. maybe you got to try to learn how to get out of the fragment from triplet to 16th from within itself.
you are likely getting thrown off when the first initial down stroke picking isnt locking in on the first division of the beat grouping. soundwise, maybe think of it as like 12 notes as one entire thing to try to get the flow in 3 beats. then on the fourth you are back to the downstroke on the first division of the beat grouping.
Video! Video! 
I totally know what you mean though
PS: suggestion pre-video → you could try to displace the accents and see if that improves the situation. For example, you can try with the first note you play being 1-16th before the beat, just after the beat, etc.
I practice it in chunks and make sure I know which finger needs to land on the beat.
It’s trickier than it sounds, especially as the tempo goes up…
Great tips, thanks guys!
I’ll sort a video soon - Ascending a 3NPS scale in 16th notes is fine but coming back down again I think the pull-offs make me rush a little and it tends to sound triplet-ty
i think to really understand that fragment down, up, pull off you need to understand rest stroke as that kinda comes from the gypsy jazz rest stroke picking realm to navigate musically descended string changes. as it is leaving out the downstroke, rest pick, hammer on/pull off, unrest pick, upstroke fragment. it is only half of the technique language for that maneuver.
because the fragment i am talking about will help you to hear the rhythm better when working on phrasing because the rest forces you to listen to what is going on before unresting to upstroke.