Crisper - I lifted the definition from a US flying school website and it is either is error or perhaps is saying that if you want to fly in US Class G airspace by definition you are VFR because it only seems to exist at low level. Here is the US AIM definition with VFR and IFR requirements:
Section 3. Class G Airspace
3-3-1. General
Class G airspace (uncontrolled) is that portion of airspace that has not been designated as Class A, Class B, Class C, Class D, or Class E airspace.
3-3-2. VFR Requirements
Rules governing VFR flight have been adopted to assist the pilot in meeting the responsibility to see and avoid other aircraft. Minimum flight visibility and distance from clouds required for VFR flight are contained in 14 CFR Section 91.155.
(See TBL 3-1-1.)
3-3-3. IFR Requirements
a. Title 14 CFR specifies the pilot and aircraft equipment requirements for IFR flight. Pilots are reminded that in addition to altitude or flight level requirements, 14 CFR Section 91.177 includes a requirement to remain at least 1,000 feet (2,000 feet in designated mountainous terrain) above the highest obstacle within a horizontal distance of 4 nautical miles from the course to be flown.
To answer your question ATC will separate the IFR aircraft in the Class E airspace containing the instrument approach. Certain classes of IFR aircraft can cancel their IFR flight plan and proceed VFR, hence in Class E airspace, cease to be separated. Other classes of IFR aircraft, or in accordance with company rules, never cancel IFR. However when they reach the instrument approach minima they become visual to perform a circling approach and are now in Class G airspace. (Note, as I wrote previously if the instrument approach is an ILS then the Class E extends to ground level, effectively a Class E control zone) ATC have rules to ensure that IFR aircraft are talking on the CTAF at non-towered airports:
4-1-5. Communications Release of IFR Aircraft Landing at an Airport Without an Operating Control Tower
Aircraft operating on an IFR flight plan, landing at an airport without an operating control tower will be advised to change to the airport advisory frequency when direct communications with ATC are no longer required. Towers and centers do not have nontower airport traffic and runway in use information. The instrument approach may not be aligned with the runway in use; therefore, if the information has not already been obtained, pilots should make an expeditious change to the airport advisory frequency when authorized.
So that was the long answer, the short one is that if the Class E airspace base is 700 feet, typically an NDB or VOR based approach, then the airport is in Class G airspace. If the approach is an ILS then the airport is in Class E airspace. There is no Class F airspace in the USA that I am aware of.
To get back to the thread, can a unicom give the information the pilot requires? This is the US AIM section:
Information Provided by Aeronautical Advisory Stations (UNICOM)
1. UNICOM is a nongovernment air/ground radio communication station which may provide airport information at public use airports where there is no tower or FSS.
2. On pilot request, UNICOM stations may provide pilots with weather information, wind direction, the recommended runway, or other necessary information. If the UNICOM frequency is designated as the CTAF, it will be identified in appropriate aeronautical publications.
UNICOM Communications Procedures
1. In communicating with a UNICOM station, the following practices will help reduce frequency congestion, facilitate a better understanding of pilot intentions, help identify the location of aircraft in the traffic pattern, and enhance safety of flight:
(a) Select the correct UNICOM frequency.
(b) State the identification of the UNICOM station you are calling in each transmission.
(c) Speak slowly and distinctly.
(d) Report approximately 10 miles from the airport, reporting altitude, and state your aircraft type, aircraft identification, location relative to the airport, state whether landing or overflight, and request wind information and runway in use.
(e) Report on downwind, base, and final approach.
(f) Report leaving the runway.