I was confused at first because i mistook the SR2 term Flesh Form with what it means in SR4.
Hybrid Forms (which where formerly known as Flesh Forms) have actually become a whole lot more powerful when compared to their SR2 equivalent.
Unfortunaltely, Flesh Forms have not benefitted to the same degree.
While the changes make bugs as a whole a much greater threat (SR2 hybrids where not more than cannon fodder, decrepit grunts with lowered stats and mostly useles powers), it has undoubtedly made the gang you mention a whole lot less plausible indeed.
Flesh Forms are by now mostly useful for infiltrating metahuman society, whereas direct combat is not the field they would excel the most in.
Let's see what we can do to enable said gang to function in the way it was intended back when Bug City originally came out.
If we take a look at inhabitation, we find the following :
QUOTE (Street Magic P.100)
Flesh Form
A spirit which attains a good merge with
its vessel becomes a near perfect flesh form.
The combined entity retains all of the memories,
abilities, and skills (both Active and
Knowledge) of the host, and its appearance
is virtually indistinguishable from that of the
original vessel. A flesh form spirit is a dualnatured
creature (p. 287, SR4), has Immunity
to Normal Weapons (p. 288, SR4), any of the
vessel’s natural and augmented abilities, and
also gains the powers of Realistic Form (p. 102)
and Aura Masking (p. 98).
The question here is what "abilities" means, as this term is not defined anywhere in SR.
Does it refer to critter powers of a potential host?
Things such as flight, increased movement speeds, natural abilites like the enhanced senses of certain animals?
Positive qualities?
All of them?
Would a spirit gain qualities such as Adept or Magician?
Normally, these would be supressed because the host's Magic attribute is replaced by that of the spirit- but the unclear implications of the term ability makes me wonder wether this is handled differently here.
The issue has been discussed in another thread, but there was neither a consensus among forumites nor any helpful comments from the devs as far as i can remember.
In any case, the best way to make your NPCs a significant thread without houseruling is to choose hosts which are already dangerous enough to harm a hybrid form spirit before they are posessed.
Undoubtedly, these are the kind of hosts a gang of female Mantid Spirits would look out for.
Also, one should utilize their spirit powers to the fullest.
Mantids posess Energy Drain (Force, Insect Spirits only, physical damage) and can convert drained Force to Karma at a 1:1 ratio, using it in the same way as free spirits.
This means that they can become very powerful over time if they hunt down enough low-level bugs.
It would easily be possible to give them powers such as Regeneration in the process.
Edit :
To further elaborate on this, here's what Street Magic has to say on free spirits and karma.
I've taken the liberty to emphasize the parts most interesting for your purposes :
QUOTE (Street Magic @ Pp 107-108)
A free spirit can use Karma in several ways at normal character
costs (see Character Improvement, p. 263, SR4). Spirits normally
have no racial maximums on their skills or attributes.
• It may raise its attributes separately (which all begin at the Force
it had when it went free).
• It may raise or purchase additional skills.
• It may initiate as a magician does, though the process grants
one additional free spirit power per grade rather than a
metamagic technique
• It may raise its Force by 1 point at a time at the cost of new Force
rating x 10. Raising Force raises all of the spirit’s attributes and
powers that are based on Force.
Given these possibilites, i wonder wether F7 is really that appropriate for the leader of a Mantid gang that has hunted other bugs for about 15 years now and has apparently rarely wasted drained Force to create new spirits.
Nymph Spirits can also be competent illusionists, whereas Soldier Spirits of a sufficiently high force can be formidable snipers.
A F7 Mantid Spirit Flesh Form Soldier doesn't have to get into melee with a bug- she just pumps him full of SnS or APDS.