Can carboxylic acids be used in the aldol reaction?
Carboxylic acids form enolates

I think I am right in saying that carboxylic acids do not form enolates because the carboxyl proton is much more acidic and so this reaction could not take place under basic conditions...

They do, if you beat them to it:

Dissolve the carboxylic acid in a polar, aprotic solvent, such as 1,2-dimethoxyethane, 1,4-dioxane, tetrahydrofuran, etc.

Add one quivalent of sodium hydride to deprotonate and form the carboxylate.

Add one equivalent of a strong base, such as LDA (lithium diisopropylamide) to deprotonate the carboxylate at the $\alpha$-position.

Add your aldehyde


What about acyl chlorides?
Adding a base, such as Hünig's base (plus catalytic amounts of a Lewis acid) to an enolizable acyl chloride under anhydrous conditions gives rise to a different pathway: a ketene can be formed!
How do ketenes react with aldehydes?
In the presence of a Lewis acid, they form $\beta$-lactones!