Have they been vocal since the start? It sounds to me that mom indeed does not have enough milk. The one that is obviously smaller is probably the runt of the litter and you should keep an extra eye on him; but the rest also need enough milk of course.
Try weighting the kittens daily. Moms are usually okay if you give them some of their favorite special food and weight the kitten close to them while she eats it. They should gain around 10-15 grams a day. Their current weight are probably somewhere between 120-140 grams if they have been eating properly.
It is important you feed mom kitten food. So try switching to kitten kibbles and kitten wetfood so she gets enough for her milk production to get up. When a mom here is short on milk I always give Hills A/D to boost her and help feeding the kittens when needed.
The fighting among kittens for 1 specific nipple isnt strange. But they should seem nice and content when done drinking and not scream a lot.
It took my cat's kits awhile to work out whose nippel was whose and the one who got the bottom middle nipple always struggled with the one that took a front one that one did eventually make lots of milk as it's kitten was the heaviest butI do think it took longer, at the same time the unused one atrophied...
It can take awhile for( day or two) mature milk to come in
Beatrix was a very small 6 pound cat prior to pregnacy she ate 7 to 9 meals of wet and or salmon or chicken bits I save from our food, and all the kitten dry she'd want ,fancy feast sized tin, the little ones would be two meals as she could only eat so much at once, the meal size went up as the kittens got bigger until they were eating large amount of wet themselves the I tapered her to 4 then two meals around the time kittens went to 2 around 4 months, then everyone got the same 2 wet all the dry kitten they wanted. Fat little guys they are now.