Angelo Custode was a small 17th century confraternity church in the street of the same name in the rione Trevi, north of the Palazzo del Quirinale. This street is now part of the Via del Tritone. A picture of the church on Wikimedia Commons is here.
The dedication is to the guardian angel of the individual believer, not to guardian angels in general, and hence is in the singular.
This edifice was designed by Felice della Greca (1625-77), a native Roman architect, and completed in 1624. A completely new church, it was intended as the headquarters of the Archconfraternity of the Guardian Angels (Archiconfraternita degli Angeli Custodi). This it remained until it was entrusted to the Society of Priests of the Sacred Heart of Jesus of Betharram, or the Betharram Fathers, in 1905.
Back then, the Via del Tritone used to run only from Piazza Barberini to Largo del Tritone, but in the 1920's it was extended to the Corso as a major traffic artery. This entailed the widening and renaming of the Via del Angelo Custode, and the demolition of the church on the south side in 1928.
Recently, the oratory of Santissimo Sacramento al Tritone which is nearby was renamed Oratorio dell'Angelo Custode in order to continue the devotion.
The Betharram Fathers are now at Santa Maria dei Miracoli.
The site is occupied by a large modern building, and the section occupied by the Banca Popolare di Milano is where it was. The entrance to the church was where a public clock on a pole now stands on the pavement; the façade was at the present kerbside or slightly into the roadway.
The plan was actually elliptical on the major axis, although the ellipse was very nearly circular. There was a separate apse on the plan of a chamfered square, and no entrance foyer. The three entrances led straight into the round nave.
The facade was designed by Mattia de Rossi, and was unusually grand for a church of this size. It was of a single storey, and had four gigantic Composite columns in the half-round, supporting an entablature with a very wide but blank frieze. This was cut through by square pilasters which continued the columns above their capitals. Above the entablature cornice was a small triangular pediment, only as wide as the space between the inner two columns. It was supported by the corresponding inner pilasters, and the central part of the cornice below the pediment was recessed to the line of the facade. The tympanum of the pediment was blank, and deeply recessed which indicates that some sort of relief sculpture was planned.
There were three entrances, the central one of which being very large. This had a a raised and strongly protruding segmental pediment above the door, and a large rectangular window above that which touched the architrave of the entablature. The side entrances, in between the pairs of semi-columns, had floating cornices and a pair of windows above, a square one above a rectangular one.
The interior was richly decorated, especially on the orders of Pope Clement XI (1649-1707) who was a member of the confraternity. There were two side altars.
The main altar was by Carlo Rainaldi of 1681, displaying a painting of the Guardian Angel by Giacinto Brandi. The left hand altar was dedicated to St Anthony of Padua, and had a portrait of him by Luca Giordano. The right hand one was to Our Lady, and had a painting depicting Our Lady Granting the Rosary to St Dominic by Carlo Maratta (or his school, more likely).
Italian Wikipedia page (The title is wrong.)