The short answer is Yes & No; but let me explain a bit further:
Both Siyalatas and Libertas are key to an idle play style; so you're going to want to beef these guys up no matter what. The question then really becomes "When is a good time?"
The calculators linked in Texenox's post are definitely helpful (I've used them as well). However; they seem to break down a bit when you start getting ancients like Iris & Morgulis.
What you need to balance is the number of hero souls you currently have vs. what it would take to upgrade. As it looks like you're pretty early game; I probably wouldn't dip below 50HS in order to buy/upgrade ancients. If you have more than that; then go for it. (although I probably wouldn't pump 100 levels into him yet).
What's been working for me is that every time I ascend; I spend 50% of the Hero Souls I get on new ancients/upgrades. That seems to give me a pretty good balance between the HS DPS bonus & upgrades.
You're probably going to want to wait on investing in Morgulis; he's better mid/late game; where you are; your HS are better spent elsewhere for the time being. There's no hidden benefit to Siyalatas (his boost actually decreases the higher he goes (-1% every 10 levels); but keep in mind this game is all about ratios; trading 5% of your HS DPS to get +25% idle damage is well worth it.
Yep; you got it. Siya is another multiplier. Basically its DPS * HS Bonus * Siya bonus. That's why it's important to try and keep things balanced. An over simplified example is that if you have 10 "things"; 8 * 2 is worse than 5 * 5. I've updated my post to point to the Siya wiki page; the discussion in there is much more in depth as to when/why to upgrade. What you can always try is to look at your DPS; spend a level on Siya; and then see if you went up or down. If you went up; spend another level. If you went down; ascend a few times before spending any more levels. Even if you go down; the # of HS spent should be easily recouped within an ascension or two.