I disagree with this. It does make a lot of difference. LRMs don't mix well with other weapons because their extreme range AND indirect fire, which allows to put them out of harms way, downgrade armor and focus on persistent damage over several turns in a row if needed. But SRMs have the same range as MLs and there isn't any issue mixing them both, having the same range requiring LoS.In general, no. No matter if it's a LRM boat or a SRM brawler, you want as much weight as possible to go towards launchers and ammo. And when you start stacking launchers, you won't have the heat capacity to run LLs anyway. For dedicated LRM boats, you could even dispense with some of the MLs as well in favour of more LRM tubes (they should never have to fire those MLs anyway, if you don't mess up your tactics).
In fact if you have enough hardpoints it may be a good idea to downgrade SRMs (for example SRM6 to SRM4) in order to add MLs, as these are more efficient with Precision Shots.
And PPCs... I wouldn't include them in any early game setup. Just too hot.