it probably wont put them off lars but they dont need next to nowt,so if you put twenty bolies in the odds in winter are massively against you ,if the water temp rises considerably they will mayb wont more,its easier to put it in than take it out.
i dont feed 20 boilies in summer
feeder fishing : maggot feeder works for barbel in winter as i reckon it attracts smaller fish and they're activity encourages the barbel to feed.
also i reckon the movement of the maggot eventually gets the barbel to respond without overfeeding them.
location: as been stated many times before i'd only think about fishing a maggot feeder if i was confident that the swim held resident barbel.
easier said than done

it boils down to experience and time spent on the bank.
give em nowt(my favourite):i personally use a boilie/paste wrap, pellet would do if thats what your confident with! fish this on a light bomb!
this way you can search the swim or swims to find where the fish are holding up without putting bait everywhere!
timing: this can differ so much early autumn 30 to 45 mins but late autumn/winter i'd be looking at 45 mins to hour an half

this is in swims where i know they live and i reckon im casting within 10yd of them possibly less!
if no bites come i'd recast 10 yd or so downstream and continue the process, this way you suss out where they lie up.
this should of come under location which is by far the most important of all the above especially this time of year.
men from the boys:
there's anglers out there who do well in summer fishing feeders pick a peg get the bait in and the fish come(not always) these are feeding fish actively hunting for food!
fish the same peg in winter and you could be 50 yd or more from where they actually live or lie up when not actively feeding(you've no chance)
i'd like to know where they came from? so i'd fish bomb and bag give it 20/30 mins then move and that way you'll find where they lie up! (summer/autumn)
if you'd actually fished the same stretch not as static you'd learn where they actually live and this information can be used for later when they're in a torpid state(not actively feeding)