Hello Stuart
hommes à pieds is infantry. Spelling at that time still fluctuates E.G; at the siege of Therouanne you have the compagnie of m. de Grutere is actually de Gruuthuse family from Bruges.
so monsieur de Sercu is probably monsieur de Sercus (a local picard nobleman) arms : silver with a sable lion (contourné = looking in the other direction) with a label of points gules
monsieur de Hely is Guillaume de Pisseleu, lord of Heily (also a local Picard nobleman) and colonel of 1000 men under Louis XII. His daughter Anne was a mistress of François I who created her Duchess of Etampes. Arms silver 3 lions gules
hope this helps
Carl