I have quite a bit of experience with Ecuador - just spent two weeks of vacations there this year too.
There are two/three agencies that you can contact in Quito - they'll put you in touch with guides. Apparently, there's a pool of guides, and only a handful of agencies that source them.
1) A few years back, I made friends with Jaime Gallardo of Zona Verde (
https://www.facebook.com/zona.verde1). He's an excellent guide - took me to the top of Cotopaxi and to many other places - he can put you in touch with good and trustworthy guides as well if he's not available.
2) You can also contact Ruta Cero (
http://www.rutacero.com.ec/). I've been on an excursion to Imbabura with them - I liked their professional style. They run a program of group excursions and organize private climbs too.
3) The last option I'll indicate is Paypahuasi:
http://www.paypahuasi.com/ - I never had the pleasure of using their services, but they seemed to be very attentive in their correspondence with me (they didn't have anyone available for the dates/activities I was looking for).
One additional word: not sure how good your Spanish is - being fluent in Spanish helps an awful lot in Ecuador in general (and with the mountain guides too).