Should I replace Armor Seals for Scaling HP Seals?

Depends really. HP can be better against another mage who won't auto attack much, but armor seals should be used against an AD heavy comp or AD mid like Yasuo/Zed.

I think flat hp seals are better than armour seals now, not sure about scaling hp.

I almost never used scaling hp unless I am running champs like Sion who particularly scales with hp and all that, otherwise I just go between flat hp or flat armour.

Here's my rune setup for mid lane

AP VS AD

Marks: MPen Seals: Flat Armor Glyphs: Scaling AP Quints: Flat AP

AP vs AP

Marks: MPen Seals: Scaling HP Glyphs: Flat MR Quints: Flat AP

The reason for this is I look at who I'm going to be fighting most in lane, if it's AP increasing levels of health as you level are great paired with the MR to help you survive the skirmishes and give you an edge, you won't get very much out of a flat 9 armor for the most part. Now if your matchup is AD, you get to scale harder with increasing levels of AP as you level up, and the armor helps you survive the lane and is pretty much necessary.

Furthermore, if there's a matchup that you know you'll win easily, like say I'm LeBlanc against Orianna, I'm going to be able to abuse that matchup pretty heavily without much problem. In a case like that I'll take both scaling HP seals and scaling AP glyphs because I'm looking more towards later on and I believe for both they beat the flat runes at level 7 give or take. It just gives you the potential for extra survivability as at level 18 you'll have a free 216 HP and 28 extra ability power, it may not seem like much but it makes a difference, the HP especially.

I think it really depends on your opposing laner. As a mid laner, you might get other AP mids, in which case i'd say take scaling health. Or you might get AD assassins, in which case you probably need early armor to stay alive :)

As someone that's super cheap, I generally build "generic" rune pages that I can use on multiple different lanes/champions to "get by".

The way I look at it is, those armor seals are going to be helping against both AP and AD mid laners because at early levels a lot of harass from your opponent is going to be auto attacks, which will be reduced by armor. Whether the defensive benefit of armor seals vs health seals is different I don't know, but I had the armor seals already from last season and id rather buy more champions to better suit team comps rather than invest a ton of RP on some runes that might not even be all that much better.

Flat health. Maybe even some mana (with the changes to %regen on items they are a lot better than they used to)

Although I still recommend 5 HP 4 armor, creep damage and auto harass is still a thing.

For an ap mid I'll do that on champs I plan to get a Zhonya's early on. If you're up against an ad mid, get the zhonya's first (or at least the cloth).

Naturally if you have plenty of IP you can have both.